It's not clear what sort of lattice you're looking for. Lattices (posets) on unlabeled nodes are at oeis.org/A006966 -- on labeled nodes are oeis.org/A055512 $$ $$ I found this by putting the word "lattice" and the first few terms (computed by hand) into the OEIS: oeis.org/Seis.html
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JBLOct 30 '10 at 15:20

(Part of the reason it's not clear is that for either interpretation, at least one of the bounds you've written down is wrong.)
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JBLOct 30 '10 at 15:30

Ah, I see, it's just that $n \leq 2$ and $n \leq 4$ are meant to be $n \geq 2$ and $n \geq 4$.
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JBLOct 30 '10 at 18:12

I mean lattice as poset in which any two elements have a unique supremum and infimum.
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tomas.langNov 1 '10 at 13:37